Chapter 7
CURRENTS, CLIMATE, AND WIND
I. Currents: movement of large masses of water through the oceans
A. Wind-driven circulation: surface currents in upper waters, above the pycnocline (<1000
m), caused by wind; major role in transporting heat from tropics to poles; 2% of wind’s
energy is transferred to ocean currents; can travel at 10-20 km/hr
1. equatorial currents: found in all the oceans just above and below the equator; driven
by trade winds
2. equatorial countercurrents: found at the equator in all oceans; flow east because water
piles up in the west due to equatorial currents and is pulled back down by gravity
3. western boundary currents: move away from equator; caused by deflection of
equatorial currents by continents and the Coriolis effect; fast, narrow, warm
a. Kuroshio Current (N)
b. East Australia Current (S)
c. Gulf Stream (N)
d. Brazil Current (S)
e. Agulhas Current (S)
i. Agulhas retroflection: abrupt turn of the Agulhas current as it meets the strong
Antarctic Circumpolar Current near the southern tip of Africa
4. eastern boundary currents: water flowing back to equator; completes gyre’s flow;
slow-moving, broad, cold
a. Peru Current [Humboldt Current] (S)
b. California Current (N)
c. Benguela Current (S)
d. Canary Current (N)
e. West Australia Current(S)
5. subtropical gyres: large swirling vortexes of water found in every major ocean basin;
caused by deflection of western boundary currents by the coriolis effect and wind
forces; rotate clockwise in the northern hemisphere and counter-clockwise in the
southern hemisphere
a. Sargasso Sea: in the middle of the North Atlantic gyre; named for large amounts
of Sargassum seaweed
b. subtropical convergence: water in subtropical gyres is driven towards the center
by Ekman transport, so it piles up in a massive hill
6. subpolar gyres: smaller swirling vortexes of water driven by polar easterlies and the
Coriolis effect; rotate opposite to the subtropical gyres in their hemispheres
7. upwelling: vertical current caused by the horizontal displacement of surface water
away from an area; occurs because volume must be conserved; brings cold, nutrientrich water to the surface, which allows plankton to survive; best developed in the east
Pacific Ocean along the west coast of South America
a. equatorial upwelling: occurs because western boundary currents diverge at
equator
b. coastal upwelling: occurs along margins of continents because Ekman transport
drives surface water away from the coast if wind is going in a certain direction;
common along west coasts of North and South America
8. downwelling: occurs in conditions opposite of those for upwelling, especially at the
poles, causing thermohaline circulation
9. Antarctic Circumpolar Current [West Wind Drift]: largest-volume current (130 Sv) in
the world; flows from west to east; driven by strong westerly winds
a. Drake Passage: squeeze of the ACC between Antarctica and tip of South America
10. Langmuir circulation: vertical circulation cells near the surface of the water caused by
steady wind blowing across calm water; the diameter of each cell (distance between
windrows) increases in higher wind speeds
a. windrows: long bands of visible plants and organic material parallel to the wind
direction; form because the circulation cells converge linearly
B. thermohaline circulation: vertical currents at high latitudes associated with the
sinking/downwelling of dense, cold, salty water (water gets saltier because when water
freezes, saltwater is excluded from ice), which spreads slowly at low depths; most
vertical circulation and mixing occurs in this way because there is no pycnocline at high
latitudes, so water can mix more easily than it would at low latitudes; important because
it mixes shallow, oxygenated water with deep water
1. North Atlantic Deep Water [NADW]: water sinking in Norwegian Sea and heading to
the North Atlantic; important to biological productivity in the Antarctic, because it
accumulates nutrients on the sea floor and surfaces them in the Antarctic
2. Antarctic Bottom Water [AABW]: water from Weddell Sea in the Antarctic; this is
the densest water in the open ocean
C. deep circulation: water spreads around the ocean floor after it sinks due to thermohaline
circulation and eventually surfaces again; can travel at 10-20 km/yr
D. associated terms and concepts
1. current flow speeds: the speed is affected by the cross-sectional area the current
travels through and the wind speed; if a current has to flow through small space, it
goes faster
2. Sverdrup [Sv = 1 million m3/s]: common unit for measuring current flow rates
3. flowmeter: instrument used for measuring the velocity of water using a propeller or
rotor
4. drift bottles: radio-transmitting devices that float with currents to provide information
about the currents
5. convergence: water flows towards other water and causes downwelling
6. divergence: water flows in opposing directions and causes upwelling
7. global conveyor belt: a worldwide connection of surface and deep currents that is
driven by wind and thermohaline circulation
8. ocean acoustic tomography: mapping features of ocean circulation by transmitting
sound and measuring its speed, which is affected by differences in salinity,
temperature, and density; speed changes show boundaries between currents
9. Coriolis effect: particles are deflected right in the northern hemisphere and left in the
southern hemisphere; maximized at the poles; zero at the equator; increased effect
with greater velocity; Coriolis force is apparent, not true
10. westward intensification: western-boundary currents are faster and narrower than
eastern ones, and the apex, or highest point, of the gyre hills are found closer to the
western boundary of a gyre, not in the center
a. dynamic topography: the comparison of different densities around the ocean; can
be used to figure out where the apex of the subtropical hill is
11. geostrophic current: net effect of the balance between the Coriolis effect and gravity,
water piled up by subtropical convergence circulates around the hill
12. Ekman spiral (Walfrid Ekman, 1902): progressively increasing deflection of surface
water with increasing depth due to the Coriolis effect; each lower layer of water is
moved slower than the one above it; 45° angle between surface flow and wind
direction; goes right in Northern hemisphere, left in Southern hemisphere
a. Ekman transport: net flow of water 90° to the direction of wind
b. depth of frictional influence (avg. 100 m): depth of no more motion
13. Hansa Carrier: shipping vessel that lost many shoes in a storm in 1990, revealing
information about the California Current near Washington when many shoes washed
up on shore
14. Pacific warm pool: wedge of warm water on the western side of the Pacific Ocean
due to trade winds
15. Antarctic Convergence (55°S): turbulent boundary where cold water meets warm
water, bringing up nutrients, which are utilized by phytoplankton
E. other types of currents
1. turbidity currents: fast-moving, sediment-laden water moving down an underwater
slope and depositing sediment that forms turbidites; this results in graded bedding
(rocks layered on top of each other based on size; small on top, large on bottom)
2. abyssal storms: fluctuations in bottom currents caused by eddies at the surface
3. eddies [vortexes, rings]: circularly flowing water offset from large currents; caused by
the meandering (bending) of a strong current as it pushes past calmer water; lead to
significant amounts of warm water being removed from the Gulf Stream
a. cold-water eddies: break off from cold currents; can be 500 km across and reach
depths of 3.5 km; high productivity due to higher levels of nutrients; spin
counterclockwise south of the Gulf Stream
b. warm-water eddies: break off from warm currents; can be 100 km across and
reach depths of 1 km; spin clockwise north of the Gulf Stream
II. Wind
A. named for the direction the wind comes from; for example, a northwesterly wind blows
toward the southeast
B. squall: a sudden, unexpected increase in wind speed due to surrounding storms
1. white squall: a dangerous squall in which there are no associated dark clouds; very
rare at sea
C. Beaufort scale: a measure of wind speed based on observations of the sea conditions;
developed by Sir Francis Beaufort, a Royal Navy officer, in 1805 in order to standardize
subjective wind speed records
D. global wind patterns
1. convection cells: air rises and falls at certain latitudes due to temperature variation,
then circulates horizontally
a. Hadley cell: 0-30 degrees north and south
b. Ferrell cell: 30-60 degrees north and south
c. Polar cell: 60-90 degrees north and south
2. wind: movement of air from high to low pressure caused by uneven heating of the
Earth’s surface; these are wind belts:
a. trade winds: 0-30 degrees north and south
b. prevailing westerlies: 30-60 degrees north and south
c. polar easterlies: 60-90 degrees north and south
3. boundaries
a. intertropical convergence zone [ITCZ, doldrums] (5°N-S): low pressure, low
wind, high precipitation zone around equator between both belts of trade winds
b. horse latitudes (30°N/S): high pressure, low wind, low precipitation zone between
trade winds and prevailing westerlies; named because sailors had to bail horses
when they didn’t have enough drinking water
c. polar front (60°N/S): cloudy, stormy, high precipitation zone between prevailing
westerlies and polar easterlies
E. land and sea breezes
1. land breeze: comes from land, heads towards ocean, occurs during night; land cools
off faster than the water, so air descends above land
2. sea breeze: comes from sea, heads toward land, occurs during the day; land heats up
in the sun faster than the water, so air rises above land
F. strong southern hemisphere winds: unusually strong because there is not much land to
slow them down
a. roaring forties (40°-50° S)
b. furious fifties (50°-60° S)
c. screaming sixties (60°-70° S)
G. other terms
1. leeward: away from the wind
2. windward: toward the wind source
3. isobar: line of constant pressure; winds travel from high-value isobars to low-value
isobars (high to low pressure)
III. Climate and Weather
A. cyclones: swirling low-pressure systems; called hurricanes in the Atlantic Ocean,
typhoons in the Pacific Ocean, and cyclones in the Indian Ocean; rotate counter-
clockwise in the northern hemisphere and air moves upward; anticyclones (high-pressure
systems) do the reverse
1. fed by warm water
2. Saffir-Simpson Scale (1-5): measures intensity of a hurricane by its wind speed
3. start out as tropical cyclones that form when the energy released by the condensation
of moisture in rising air causes a positive feedback loop over warm ocean waters
4. eye wall: region just outside of the eye where winds are most damaging and rainfall is
greatest
5. storm surge: high water below the low-pressure eye of a hurricane that can be
disastrous along the coast
6. Galveston, Texas: a hurricane hit this island in 1900; this was the single deadliest
natural disaster in US history
7. Hurricane Katrina (August, 2005): hit Gulf Coast and broke New Orleans levees
8. global warming will increase the frequency of strong hurricanes, but there will be
fewer hurricanes overall
9. waterspout: tornado over the ocean that is common around the edges of cyclones
B. monsoons (Arabic mawsim=season): season of winds that brings lots of rain to Southeast
Asia from air masses above the Indian Ocean
C. tornadoes
1. intensity of a tornado is measured by the Fujita Scale (F Scale) normally from 0-5
D. greenhouse effect: infrared light emitted from Earth’s surface is absorbed by greenhouse
gases in the atmosphere, so heat is retained on Earth; this is the cause of global warming
and global climate change;
1. top greenhouse gases (by total contribution, not amount):
a. water vapor [H2O]
b. carbon dioxide [CO2]
c. methane [CH4]
d. ozone [O3]
e. nitrous oxide [N2O]
f. CFCs
2. Intergovernmental Panel on Climate Change [IPCC]: group of scientists studying the
human effects on global warming
3. Kyoto Protocol: agreement of nations to limit greenhouse gas emissions
E. solar radiation
1. more light is absorbed at low latitudes than at high latitudes, because the angle of
incidence of sunlight is closer to being perpendicular to Earth’s surface
2. sunlight is always concentrated between the Tropic of Cancer (23.5°N) and the
Tropic of Capricorn (23.5°S)
3. unequal heating of Earth leads to currents and wind transferring thermal energy
4. heat budget: balance of heat across Earth's surface
5. albedo [reflectivity]: the amount of solar radiation reflected by a certain area of the
Earth; higher for ice than it is for water
6. insolation: the amount of solar radiation received by a certain area of the Earth in a
certain amount of time
a. Langley [Ly = J/m2]: standard international unit for measuring solar radiation
intensity, absorption, or albedo; has units of energy per area
F. El Niño Southern Oscillation [ENSO]: climatic anomaly of the equatorial Pacific due to
variations of the Walker cell, an atmospheric circulation cell in the equatorial South
Pacific
1. El Niño [Spanish = “the little boy”]
a. lasts 1-1.5 years and starts during Christmas
b. high temperatures, and thus a deeper thermocline in the eastern Pacific
c. weak trade winds
d. less upwelling, which hurts the fishing industry
e. more storms in central Pacific
f. floods in west South America
g. drought in west Pacific and northern Brazil
2. La Niña: sometimes occurs after El Niño; characterized by:
a. lower temperatures
b. stronger trade winds and stronger Walker cell circulation
c. more upwelling in eastern Pacific, which helps the fishing industry
d. more Atlantic hurricanes
3. Tropical Ocean Global Atmosphere [TOGA]: program designed to study how El
Nino events develop
G. Pacific Decadal Oscillation [PDO]: phenomenon of varying Pacific sea surface
temperatures every 20 to 30 years
H. North Atlantic Oscillation [NAO]: phenomenon of varying Atlantic sea surface
temperatures; less temperature variation than PDO
I. Other
1. dewpoint: temperature at which the air is saturated with water; water condenses as
dew or fog below this temperature
2. meteorology: the study of weather and climate